What is an LCD Monitor?

The basics:

The LCD Monitor on a Canon EOS camera is the larger panel on the back of the camera, as opposed to the PCD panel on top of some EOS models.

A Little More Detail:

The LCD Monitor is used to view the menus and camera settings, playback captured images and videos, and also frame your shot (when using Live View). Some LCD Monitors are also Touch-Screens, making it easier to interface with your camera.